A gas mask of the type mentioned has become known from EP 667 171 A1. In gas masks operating with overpressure, a pressure is generated within the interior space of the mask, which is to prevent harmful gases from penetrating into the mask. Gas masks with a filter at the mask connection operate under normal pressure. The opening pressure of the expiration valve must be adapted to the mode of operation by means of a changeover switch in the prior-art gas mask, which can be used for both normal pressure operation and overpressure operation. The expiration valve is received for this purpose in a valve housing in the vicinity of the breathing connection, and the expiration valve comprises a pin, which is displaceable in relation to the valve housing, a valve disk, a valve plate, which is in contact with the valve seat, and a valve spring between the valve disk and the valve plate. The pin with the valve disk, the valve plate and the valve spring are connected with one another in one assembly unit. A sensor, which is braced in relation to the valve housing with a compression spring, extends from the breathing connection into the valve housing and is in contact by its rear side with the valve disk. The compression spring and the valve spring are arranged in a series-connected arrangement, and the compression spring is softer than the valve spring.
In case of normal pressure operation, for example, with the filter inserted at the breathing connection, the sensor can move freely and the pin of the expiration valve slides within the valve housing when the valve plate is lifted off from the valve seat during expiration. Since the valve disk is supported against the sensor, the opening pressure of the expiration valve is determined by the rigidity of the compression spring and by the pretension thereof.
In case of the use of a lung demand valve for overpressure operation, the sensor is blocked and so is the pin in the valve housing via the valve disk. The opening pressure of the expiration valve is determined solely by the rigidity and the pretension of the valve spring.
The drawback of the prior-art gas mask is that the design of the changeover switch with two springs connected in series is complicated and prone to errors.
A changeover switch for normal pressure operation and overpressure operation at a gas mask is known from IT 12 27 248 B, in which the opening pressure of the expiration valve is adapted to the mode of operation by changing the pretension of the valve spring. The force with which the valve spring is pressed against the valve plate is increased for this purpose by means of a displaceable sensor when a lung demand valve for overpressure operation is connected to the breathing connection. The drawback is that a relatively long path of adjustment of the sensor is necessary for changing over from normal pressure operation to overpressure operation.